1. Field
The technology described herein relates in general to long range transmission of RFID signals over coaxial cables to transmitter antennae and in particular to a system for maintaining a desired RF signal level at the input to an antenna by substantially compensating for attenuation effects over the coaxial cable length.
2. Related Art
In one of applicant's systems, as described in commonly-assigned U.S. patent application Ser. No. 11/254,250 which is incorporated hereby by reference, a single BRT (Backscatter Receiver/Transmitter)/hub (a Spider) with antennae attached to multiple transmit and receive ports by coaxial cable may be used to radiate instruction signals to tags associated with retail merchandise displays and receive RF signals from the tags over an entire establishment, such as a drug store, as well as a sizable portion of grocery and mass merchandising stores. Entire grocery and mass merchandise outlets can be covered by a small number of Spiders.
With the Spider described above, each of a plurality of RFID tags located in spaced relationship to the other RFID tags in a retail environment can be interrogated by transmitted signals and respond by radiating information concerning commercial goods with which it is associated.
Long Range RFID systems that are capable of determining the location of RFID tags within a retail environment should have consistent and predictable performance over a variety of conditions. Such conditions include coaxial cable specifications, electronics aging, and temperature variations. This is particularly important when considering the RF signals applied to the transmitter antennae in such systems.
A predictable and consistent Effective Radiated Isotropic Power (ERIP) will aid in the tag location determination. Applicant's present display monitoring system uses a central transmitter and receiver known as the Main Electronics Unit (MEU) that connects with remote active receive and transmit antennae via coaxial cable. As is well known, RF power transmitted over coaxial cable loses strength from the input to the output of the cable. Two parameters generally define the loss for a particular type of cable. These parameters are cable length and frequency of the RF signals transmitted over the coaxial cable.
It will be understood by those skilled in the art that the lengths of coaxial cables installed in long range RFID systems can vary from one installation to the next. In one system, a transmit antenna may be connected to the MEU with a 25′ cable while in another system, an 85′ cable (or longer) may be required. Applicant's display monitoring system presently operates in the 915 MHz ISM band. A typical loss for plenum rated coaxial cable is 0.15 dB/ft. This equates to a signal loss of 3.75 dB for a 25′ coaxial cable and 12.75 dB for the 85′ coaxial cable, a difference of 9 dB. This is a huge variation and creates an undesirable variable when computing RFID tag locations.
It would be desirable to have a power tracking loop (gain control system) incorporated into the transmitter antenna circuitry to address the power variation problem.